Reference for Berakhot 15:8

Rab Hisda said : A man should always enter two doors in the Synagogue and then pray ; as it is said, "Waiting at the posts of My doors." "Two doors" [literally], dost imagine ! But say [the meaning is], A man should penetrate into the Synagogue a distance which equals the width of two doors, and then offer prayer.

Jastrow